«Proceedings of the th 14 National Street Tree Symposium 2013 ISBN: 978-0-9806814-1-3 TREENET Proceedings of the 14th National Street Tree Symposium ...»
However, the design of the “back end”, the hole in the ground that receives the water from the inlet, is very much the subject of current research. The principal requirements are that it has sufficient capacity to accept all of the most polluted first flush component and that it is accessible to tree roots for extraction of the captured water. A hole or trench of at least half a cubic meter is backfilled with a no fines aggregate and a standard 90 mm stormwater pipe conveys the stormwater from the back of the TREENET inlet in the kerb to this simple “cistern”. The open nature of the aggregate allows rapid uptake of the first flush water and retains it until it has infiltrated into the surrounding subsoil, principally at depth. Infiltration rates into the surrounding soil are improved as the roots decompact the verge and biopores are created following the senescence and replacement of fine absorption roots. One simple system installed at the Waite Institute in 2011 is illustrated below. It is called the “black hole” because it has been backfilled with approximately.75 m3 of recycled water filtration residue from Happy Valley reservoir which contains a high proportion of pollutant absorbing activated carbon.
Photograph 1. and Photograph 2. examples of TREENET inlet Figure 7. and Figure 8. TREENET Inlet Design Water Sensitive Urban Design Development Photograph 3. Vertical kerb lift resulting in ponding The 14th National Street Tree Symposium 2013 Common street trees throughout Salisbury are E. leucoxylon (var.), E. torquata, and E. sideroxylon7. When planted within narrow verges and in close proximity to kerbing (within 600mm from back of kerb), the larger species of E. sideroxylon has presented lifted and broken sections of kerb. In turn ponding of stormwater after rain events is evident. Ponding within driveway crossovers and adjacent properties are regularly reported complaints to council, raising issues regarding standing water, build-up of leaf-litter and debris. As part of maintenance works, Councils Road Reseal and Kerb Replacement Programs aim to rectify lifted and damaged kerb, correcting the grade for storm water flows. Locations are identified by visual inspection following asset audits that rank streets according to road failures. In some cases to address ponding, twenty metres of kerb replacement is usually required to achieve appropriate grade. The average cost of replacing a standard section of kerb is $130 per lineal metre with council spending $700,000 in 2012/13 and with increased roads resealing in 2013/14 $1M for kerb replacement will be expected to be invested for the 2013/14. In many locations kerbing has been replaced numerous times to prevent such ponding. This ongoing work and repetitive replacement signified a change in thinking.
An investigation into alternate methods ensued. With the following points in mind six differing methods of implementing localised infiltration/ bio-filtration sumps ensued.
Reducing ponded water within kerbing Improving soil moisture recharge Maximising contact surface area with the surrounding soil Varied methods of distributing captured water within the verge Depth of sumps relative to infrastructure and services Maximising volume capacity within sump Reduced spatial capacity for restricted verge spaces Reducing leaf litter and debris build up Minimise maintenance Identified Risks Public safety included trip hazards, saturated/ boggy areas, Vandalism Vehicle damage Proximity to infrastructure Root damage during excavation As a launching point Council required an initial test location for the six options; Orlyk Street Para Hills West was chosen as the test site. This street was selected due to the amount of locations presenting standing water within the kerb and the presence of mature street trees (E sideroxylon).
Initial installation of the infiltration sumps was undertaken by existing contractors for kerb and gutter replacement. Training was required for the contractor to understand the differing method of installing the TREENET Inlet and infiltration sumps. Continued implementation of these infiltration sumps relies upon effective monitoring and asset take-up by Council maintenance staff.
Leaf litter within kerbing is generally cleared from kerbing as part of Council’s Verge Cutting Program. The program is programmed on a 5 – 8 week cycle of grass reduction (May to December) and accompanied removal of clippings from footpaths and driveways. Street sweeping follows within 48hours of cutting activities8 collecting leaf litter and debris from kerbing simultaneously. Autumn signifies the flowering of E sideroxylon and the migration of the Cacatua galerita (Sulphur Crested Cockatoo), Glossopsitta concinna (Musk Lorikeet) and Platycercus elegans (Crimson Rosella) to the district. Large numbers of these birds foraging amongst the street trees shed leaf litter and trimmed branch-lets onto the roadway. Consequently autumn rains flush this leaf litter along the kerb. Due to the overland storm water system upon flat grades, leaf litter and debris exacerbates localised ponding to occur within kerbing. During high intensity rain events side entry pits (SEP) can become choked with leaf litter and debris, slowing down the storm water flows. This causes the SEP to back up and local flooding ensues. Council has programmed vacuum clearing of SEPs to prevent blockages; it is unrealistic to rely on this program to offset the total volume of litter during certain times of year.
To counter the build-up of leaf litter and debris in front of the Tree Inlets face plates and SEPs leaf litter collection/ infiltration bays have been developed. Infiltration bays or Rain Gardens are anticipated to reduce roadway debris with select street tree species. The design of these relies upon gravity, as its precedent the slotted kerb and swale approach does. As storm water discharges along the kerb, it flows past the infiltration bays; the open kerb causes the storm water to fall into the Rain Garden carrying with it any leaf litter or debris.
The Rain Garden is a trench of two meters long filled with 20mm aggregate screenings and the edges are battered with local spoil and planted with native grasses. Storm water discharges down through the profile of the Rain Garden, leaving leaf litter and debris upon the surface of the aggregate. Upon saturation of the infiltration bay and immediate surrounding soil, the Rain Garden at full capacity allows the storm water continue its path along the kerb; as it would had the infiltration bay not have been there. During high intensity flows, leaf litter and debris is anticipated to be continually deposited and removed to within the Rain Garden due to a naturally occurring eddy9.
8 Verge Cutting Program, City of Salisbury http://www.salisbury.sa.gov.au/Our_City/Vehicles_Parking_Transport_and_Roads/Footpaths_and_Verges/Verge_Cutting_Prog ram Eddy (fluid dynamics): In fluid dynamics, an eddy is the swirling of a fluid and the reverse current created when the fluid flows past an obstacle. The moving fluid creates a space devoid of downstream-flowing fluid on the downstream side of the object.
Fluid behind the obstacle flows into the void creating a swirl of fluid on each edge of the obstacle, followed by a short reverse flow of fluid behind the obstacle flowing upstream, toward the back of the obstacle.
http://en.wikipedia.org/wiki/Eddy_(fluid_dynamics) accessed 10/07/2013 The 14th National Street Tree Symposium 2013 Photograph 5. and Figure 11. Salisbury Council Water Sensitive Urban Design The methods explored thus far include nine designs using the TREENET Inlet and two Rain Garden deisgns consisting of various sized unlined sumps filled with 20mm aggregate, with three different ways of dispersing the inflow around the sump. The variation in size has been designed to accommodate different storm water volumes, available space within verge area including the placement of sumps below footpaths and driveway crossovers; and maximise contact surface area with the surrounding soil.
Implementation of infiltration and bio-filtration sumps has required the cooperation of Council Tree Services staff identifying in-appropriately planted street trees for removal and replacement plantings adjacent TREENET Inlet sumps. This cooperative approach has expedited the process and timing of works. Using the TREENET Inlet sumps in proximity to replanting’s has raised the opportunity to test the benefits of street tree establishment. Exploring further has led to Tree Services staff actively pursuing problematic trees for removal, replant and placement of TREENET Inlet. Previously where staff displayed apprehension in removing problematic trees due to re-establishment concerns; the TREENET Inlet has empowered staff to take positive steps towards replanting locations.
Trees and Infrastructure and the Heat Island Effect Pavement stresses can be attributed to many factors and heat being one that can be reduced by shading using the trees canopy. Salisbury Council is currently using Micro surfacing treatments, polymer road stabilisation in conjunction with WSUD.
Randrup et al. (2003) suggested that certain pavement construction methods may even promote damage to pavements by tree roots. It was explained that soil moisture loss by evaporation can be blocked by the barrier such as a concrete or asphalt pathway. Due to the evaporation barrier affect, there are differences in temperature between the soil and pavement and this causes the soil moisture to condense on the underside of the pavement. Damage is caused therefore to the pavement surface by the root growth being attracted to this condensation moisture at the soil/concrete interface. Randrup et al. (2003) proposed that pavements constructed from porous materials that limit condensation and lower the temperature under concrete slabs may reduce the incidence of rooting at the interface and the subsequent damage this can cause.
The negative environmental impacts of treeless streets (this includes enhanced heat island effects) has been researched significantly, plus continued concern about the impacts of climate change have helped sway a change in attitude toward urban street trees by town planners and designers; (Shashua-Bara and Hoffman, 2000) - it is duly acknowledged that urban street trees provided positive economic and environmental benefits to the community10.
Salisbury recognises the important role that street trees play in urban environments has launched research into developing alternate methods of passively irrigating street trees that may lower maintenance costs and promote healthier and faster growing trees.
Trees as essential infrastructure: Engineering and design considerations. pg19 par 5, Beecham S. School of Natural and Built Environments and Centre for Water Management and Reuse. University of South Australia The 14th National Street Tree Symposium 2013 Opportunities for using Water Sensitive Urban Design By implementing TREENET Inlets/rain gardens throughout the council area it is envisaged improvements in the
community’s amenities may, include but not limited to the following:
Mitigate Infrastructure damage by reducing kerb replacement works and pavement deterioration Minimise impact on established trees Improve establishment of street tree planting using water sensitive urban design Catchment of nitrates, particulates, pollutants and organics and improve soil moisture for street trees.
Reduce first flush pulses Reduce silt build up in Council Wetlands Reduce storm water volumes discharging out to sea Reduced maintenance practices by redirecting leaf fall and diverting water prior to the CoS pipe network. Work Health and Safety improvement by lessening the requirement to clean side entry pits.
Reduced impact of the heat island affect from heat pavement reflection.
Author Biographies Peter B Young B.Des.St., M.Larch Landscape Design Officer, City Of Salisbury, SA Peter’s early career was focussed on skatepark design and promotion of this form of recreation culminating in 2009 with a redevelopment masterplan for Canberra’s Belconnen Skatepark to become Australia’s largest.
During University of Adelaide studies in landscape architecture he worked with Taylor Cullity Lithean and credits a rigorous design ethos with this experience.
Since 2008 Peter has been employed at City of Salisbury working on a range of landscape and biodiversity projects.
Peter is a strong advocate for water sensitive urban design in local email@example.com government.
Peter Levett Capital Works Officer, City of Salisbury SA Peter Levett is a highly experienced practitioner in civil and landscape construction and maintenance in local government public works with major metropolitan Adelaide councils.
In his current role at City of Salisbury Peter administers a significant road surfacing program and other civil works. He has lead initiatives to achieve carbon reduction strategies and is active in cross council forums seeking improved industry standards in road surfacing.
(1) Trees as essential infrastructure: Engineering and design considerations. Beecham S. School of Natural and Built Environments and Centre for Water Management and Reuse. University of South Australia (2) Plant available moisture in stone-soil media for use under pavement while allowing urban tree root growth. Grabosky J, Haffner E and Bassuk N in Arboriculture & Urban Forestry, 35(5), 271-278. (2009)
(4) Vegetation as a climatic component in the design of an urban street: An empirical model for predicting the cooling effect of urban green areas with trees. Shashua-Bara L and Hoffman M E, in Energy and Buildings, 31(3), 221-235 (2000) (5) From The Gutter To The Stomata By The Closest “Root” David Lawry (6) The benefits of Adelaide’s street trees revisited Professor Randy Stringer (7) Are street trees and their soils an effective stormwater treatment measure? Liz Denman (8) Salisbury Development Plan (9) Sustainable Futures The Prosperous City 1.2 Support the development of a workforce possessing the skills required to adapt to industry restructuring and meet the needs of growth orientated industry sectors.